Data storage devices include those normally provided in connection with a host computer or other electronic device. In one general category, data is stored on a fixed or rotating (or otherwise movable) storage medium and a read/write head is positioned adjacent to desired locations of the storage medium for writing data thereto or reading data therefrom. A data storage device of this type is a disk drive.
Disk drives store information on magnetic disks. Typically, the information is stored in concentric tracks on the disk and the tracks are divided into servo sectors that store servo information and data sectors that store user data. A head (or transducer) reads from and writes to the disk. The head is mounted on an actuator arm that moves the head radially over the disk. Accordingly, the actuator arm allows the head to access different tracks on the disk. The disk is rotated by a spindle motor at high speed, allowing the head to access different data sectors on the disk. The head may include separate or integrated read and write elements.
FIG. 1 illustrates a disk drive 10 that includes a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted on a base plate 16. The disk drive 10 also includes an actuator arm assembly 18 having a head 20 mounted on a flexure arm 22, which is attached to an actuator arm 24 that rotates about a bearing assembly 26 that is attached to the base plate 16. The actuator arm 24 cooperates with a voice coil motor 28 to move the head 20 relative to the disk 12. The spin motor 14, the head 20 and the voice coil motor 28 are coupled to electronic circuits 30 mounted on a printed circuit board 32. The electronic circuits 30 include a read channel, a microprocessor-based controller and a random access memory (RAM). The disk drive 10 may include multiple disks 12 and therefore multiple actuator arm assemblies 18.
FIG. 5 is a top view of a disk 512 having a disk surface 542 which has embedded servo information. The disk surface 542 includes concentric tracks 544a–544h. Each track 544 is divided into data sectors 546 and servo sectors 548. The servo sectors 548 in each track 544 are radially aligned with the servo sectors 548 in the other tracks 544, thereby forming servo wedges 550 which extend radially across the disk surface 542 from the disk's inner diameter 552 to its outer diameter 554. Although a relatively small number of tracks 544 are shown for ease of illustration, it should be appreciated that typically tens of thousands of tracks 544 are included on the disk surface 542.
During a seek operation, the head 20 is moved from a present track to a target track so that a data transfer can be performed with the target track. In addition, a current is delivered to the voice coil motor 28 which causes the actuator arm 24 to rotate, thereby moving the head 20 radially relative to the disk 12.
During a track following operation, the head 20 is maintained over the target track using a position error signal (PES) based on servo information that the head 20 reads from the servo sectors in the target track.
FIG. 2 is a sectional view of the disk 12 and the head 20. The head 20 (which includes a slider) is located above a disk surface 42 by a flying height 100. The flying height 100 is created by the interaction between air currents above the disk surface 42 (also known as an air-bearing) caused by rotation of the disk 12 and the aerodynamics of the slider of the head 20.
It is important to maintain the flying height 100. For example, if the head 20 flies too low, it is more likely to contact the disk 12, which could cause stored data to be lost. As another example, if the head 20 flies too low, a particle resting on the disk surface 42 may attach to the head 20 and change the aerodynamics of the head 20.
FIG. 3 is an air-bearing surface view of the head 20 which illustrates a write portion 110 of the head 20 and a read portion 120 of the head 20. For clarity, the slider of the head 20 is not shown. The write portion 110 includes a write pole 130 and a return 135. The read portion 120 includes a magneto-resistive (MR) read element 140 along with first and second shields 142, 144. The direction of disk rotation is shown by arrow 150 such that the write pole 130 follows the read element 140.
FIG. 4 is a cross-sectional, side view of the head 20 that illustrates a write coil 155, a write gap 160 and a read gap 165. The write portion 110 writes perpendicular magnetic polarity transitions onto the disk surface 42. Perpendicular recording is well-known in the art and requires a disk that is capable of having perpendicular magnetic polarity transitions recorded thereon, for example, by including a soft magnetic underlayer.
During a write operation, a variable write current is supplied to the write coil 155 to induce magnetic flux across the write gap 160. The direction of the write current defines the direction in which the magnetic flux is oriented across the write gap 160. In simple recording systems, magnetic flux polarized in one direction across the write gap 160 records a binary one while magnetic flux polarized in the opposite direction records a binary zero. In most recording systems, a change in the direction that the magnetic flux travels across the write gap 160 records a binary one while the lack of such change records a binary zero. As the disk 12 travels under the write portion 110, a series of ones and zeros are written to the disk surface 42.
During a read operation, the first and second shields 142, 144 define the read gap 165 which focuses the magnetic flux for a particular magnetic polarity transition onto the read element 140 by shielding the read element 140 from other sources of magnetic flux. In other words, extraneous magnetic flux is filtered away from the read element 140 by the shields 142, 144. The read element 140 generates a read signal in response to the changing magnetic flux which corresponds to previously recorded data as the magnetic polarity transitions in the disk 12 pass underneath it.
The write portion 110 and the read portion 120 are located near the trailing edge of the head 20. Furthermore, the head 20 is pitched relative to the disk surface 42 such that the trailing edge is closest to the disk surface 42 (see FIG. 2). Since the write portion 110 trails the read portion 120, the write portion 110 (specifically the write pole 130) is closest to the disk surface 42. In addition, the write pole 130, the return 135, the read element 140, the first shield 142 and the second shield 144 share a common plane 175 at an air-bearing surface which faces the disk surface 42.
Disk drives usually store information on disks using longitudinal recording as opposed to perpendicular recording. However, the heads associated with longitudinal recording may be very similar to the head 20 in that the write pole, return, read element, first shield and second shield share a common plane.
Although the manufacture, distribution and use of disk drives follow a number of models, tests are usually performed following assembly of a disk drive before it is delivered to a user. The tests include performance, reliability and environmental tests. Environmental tests measure how the disk drive reacts to temperature, pressure or other environmental factors. For example, environmental tests may store information to control the magnitude of the write current as a function of ambient temperature since a high write current may be needed before the disk drive warms up.
Likewise, servo information is usually provided in the servo sectors before the disk drive is delivered to a user. Servo information includes sector markers or identifiers and track markers or identifiers and is typically used for generating the PES to position the head relative to the disk during read and write operations in which user data is received from or sent to a host computer.
The general trend in data storage devices including disk drives is higher data density on the storage medium. Higher data density permits a physically smaller data storage device for a given storage capacity and can also enhance performance (such as reducing seek times). Higher data density often requires a reduction in flying height. However, pole tip protrusion causes difficulties at reduced flying height. Pole tip protrusion refers to thermal expansion of the tip of the write pole 130 towards the disk 12 in response to the write current. At moderately high write currents during prolonged write operations, the write pole 130 may protrude sufficiently to contact the disk 12, especially if the flying height is small. Pole tip protrusion can result in data loss where the write pole 130 contacts the disk 12.
A previous approach to detecting head-disk contact involves detecting write faults caused by the PES going outside a write inhibit window. However, this approach is less sensitive than desired since head-disk contact is not always detected.
Another previous approach to detecting head-disk contact involves detecting thermal asperities, as described in U.S. Pat. No. 6,195,219. While the thermal asperity approach provides acceptable results for contact recording, there is substantial room for improvement for non-contact recording (in which the head flies over the disk).
Accordingly, it would be useful to provide an improved approach for detecting head-disk contact as well as determining the contact write current at which head-disk contact occurs or is likely to occur.